This invention relates to a workpiece holder and method for use in a plasma reactor apparatus, and more particularly to the workpiece holder and method which provide temperature control and uniformity of heat distribution to the work pieces.
Plasma reactions are finding widespread usage in a number of applications, especially in the semiconductor industry. In the semiconductor industry, for example, plasma reactions are used for etching, ashing, or the like. In etching, the plasma reaction is typicaly used to selectively pattern or etch a layer of oxide, nitride, or polycrystaline semiconductor material or to etch moats in a semiconductor wafer itself. In ashing, the plasma reaction can be used to strip a layer of photoresist or other material from the semiconductor wafer as a stripping or cleaning process. In such operations the wafers or other work pieces are loaded into a boat or holder. The boat and work pieces are placed in a plasma reactor apparatus, an appropriate reactive gas in introduced, and RF power is applied to the apparatus to form a reactive plasma of this gas which can interact with the work pieces. The holder supports and positions the work pieces to properly expose them to the reactant plasma. As the reaction proceeds, the work pieces and holder are heated by two mechanisms. First, heating is produced by thermal transfer from the molecules in the plasma. This produces a slow heating and is a function of such variables as the RF power, the pressure within the reactor, and the gas selected for the reaction. Second, if the holder is metallic, as is often the case, and the holder is in contact with one of the electrodes of the RF circuit, the holder then becomes a part of the RF circuit and is heated by Joule heating. This heating can be very rapid and can cause a temperature rise of several hundred degrees Celsius. The rate of a plasma reaction is typically a strong function of temperature, with the reaction rate increasing rapidly as the temperature increases. Because of this, uncontrolled or non-uniform heating of the workpieces can provide a serious problem.
As a specific example, semiconductor wafers are typically positioned vertically in a boat. The boat has three or four slotted or notched rails and the wafers are placed in and held by these notches. If the boat is metallic and is in contact with an electrode of the plasma apparatus, the boat heats rapidly and transfers heat from the rails to the wafer at those points where the wafer and rails are in contact. Thus, the contact points of the wafer are raised to a higher temperature than the rest of the wafer and the plasma reaction is non-uniform across the wafer. This can occasion undercutting, non-uniform etching, poor pattern definition, and the like, all of which lead to reduced yield. In addition, in some applications such as ashing it is desireable to heat the wafers so as to enhance the reaction rate and thus increase the through-put. Prior art workpiece holders have failed to consider the very important aspects of temperature control and heating.
Because of the deficiencies of the prior art workpiece holders, it is an object of this invention to provide an improved workpiece holder and method which provide for controlling the temperature of the workpieces and provide for a uniform temperature distribution across the workpieces.